1. Field of the Invention
The present invention concerns in general terms a method of determining the spatial transfer function of a transmission channel by means of an array of antennae. More precisely, when the propagation of the signal between a transmitter and an array of reception of antennae takes place along a plurality of paths, the present invention makes it possible to estimate, for each path, the direction of arrival conjointly with the attenuation and phase rotation undergone by the signal along this path. The present invention applies more particularly to the estimation of the channel and the direction of arrival in mobile telephony.
2. Description of Related Art
The propagation of a signal between a transmitter and receiver, in particular between a mobile telephone and a base station, can take place along multiple paths. Each path is characterised by a delay, a direction of arrival, an attenuation and a phase rotation.
It is known that an array of antennae can be used to form a beam and/or cancel interference in one or more directions. The antenna processing consists of a weighting of the outputs of the different antennae with complex coefficients before summing, the coefficients being chosen so as to obtain the equivalent antenna diagram required. It is thus possible to form a beam in the direction of arrival of the useful signal whilst placing zeros in the directions of arrival of the interfering signals. The majoring of the beam formation techniques however require prior knowledge of the direction of arrival of the signal. Beam formation has been applied to mobile telephony, notably to direct a reception beam from a base station to a mobile station (uplink). The base station is then equipped with an adaptive antennae (referred to as an “intelligent antenna”) capable of pointing in the direction of a propagation path issuing from a mobile terminal.
In addition to the direction of arrival of the useful signal, it is often necessary to know the characteristics of the transmission channel. Thus, when a mobile telephony system is functioning according to a code division multiple access (CDMA) mode and is exploiting diversity in reception by virtue of a rake receiver, the complex coefficient used in the different branches of the receiver are obtained from the attenuation and phase rotation undergone along the different paths.
It is known that an adaptive antenna can be used, exploiting the diversity of paths. For this purpose, the systems of the state of the art use one rake receiver per antenna and effect the beam formation from signals issuing from the different receivers. It can also be envisaged pointing several beams in the directions of arrival of the different paths and then combining the different paths in a rake receiver. In both cases the direction of arrival of the useful signal and the characteristics of the transmission channel are estimated independently. This approach has the double drawback of requiring two estimation steps and not using all the information available at the second estimation step.